Systems and Methods for Network MIMO

ABSTRACT

A multiple-input, multiple-output (MIMO) communication system comprising a master base station and a slave base station. The master base station has a plurality of transmit antennas and transmits a first set of data to a mobile station in a first transmission. The slave base station has a plurality of transmit antennas and transmits a second set of data to the mobile station in the first transmission. The master base station retransmits the second set of data to the mobile station during a first retransmission and the slave base station retransmits the first set of data to the mobile station during the first retransmission.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/932,754 filed Jul. 1, 2013 by Kelvin Kar-Kin Au, et al. entitled,“Systems and Methods for Network MIMO” (Attorney Docket No.44752-US-CNT2-4214-57303), which is a continuation of U.S. Pat. No.8,537,773, issued on Sep. 17, 2013 entitled, “Systems and Methods forNetwork MIMO” (Attorney Docket No. 44752-US-CNT), which is acontinuation of U.S. Pat. No. 8,331,308, issued on Dec. 11, 2012entitled, “Systems and Methods for Network MIMO” (Attorney Docket No.44752-US-PAT), which claims priority to U.S. Provisional Application No.60/986,846 filed Nov. 9, 2007 by Kelvin Kar-Kin Au, et al. entitled,“Systems and Methods for Network MIMO” (Attorney Docket No.44752-US-PRV), all of which are incorporated by reference herein as ifreproduced in their entirety.

TECHNICAL FIELD OF THE INVENTION

The invention relates to communication systems in general, andparticularly to MIMO (Multiple-Input Multiple-Output) communicationsystems.

BACKGROUND OF THE INVENTION

In a MIMO communication system, a transmitter transmits data throughmultiple transmitting antennas and a receiver receives data throughmultiple receiving antennas. The data to be transmitted is usuallydivided between the transmitting antennas. Each receiving antennareceives data from all the transmitting antennas, so if there are Mtransmitting antennas and N receiving antennas, then the signal willpropagate over M x N channels, each of which has its own channelresponse. The movement of the receiver in relation to the transmitterresults in significant fluctuation in channel conditions. The multipleantennas provide spatial diversity for communications. Typically, if thereceiver requires a large transmission power for data, for example areceiver that is geographically located at the edge of a communicationcell, the receiver is attended to by using a different transmission thanreceivers in closer proximity to the transmitter.

In order to improve coverage and throughput, network MIMO can be used.In network MIMO, each receiver is in network MIMO communication withmultiple transmitters.

SUMMARY OF THE INVENTION

In accordance with a broad aspect, there is provided a mobile station ina multiple-input, multiple-output (MIMO) system for enabling networkMIMO communication between the mobile station and a plurality of basestations. The mobile station comprises a plurality of receive antennasand a control system. The control system is configured to receive in aninitial transmission a first set of data transmitted by a master basestation and a second set of data transmitted by a slave base station;and receive in a first retransmission the second set of dataretransmitted by the master base station.

In some embodiments, the control system is further configured to receivein the first retransmission the first set of data retransmitted by theslave base station.

In some embodiments, the control system receives the retransmittedsecond set of data from the master base station synchronously with theretransmitted first set of data from the slave base station.

In some embodiments, the control system receives the retransmittedsecond set of data from the master base station asynchronously with theretransmitted first set of data from the slave base station.

In some embodiments, the first set of data comprises the symbols s₁, s₂,−s₁*, and s₁* and the second set of data comprises the symbols s₃, s₄,−s₄*, and s₃*, where * denotes complex conjugate.

In some embodiments, the symbols s₁, s₂, −s₁*, and Ware space-timecoded, and the symbols s₃, s₄, −s₄*, and s₃* are space-time coded.

In accordance with another broad aspect, there is provided a method in amobile station in a multiple-input, multiple-output (MIMO) system forenabling network MIMO communication between the mobile station and aplurality of base stations. The method comprises receiving in an initialtransmission a first set of data transmitted by a master base stationand a second set of data transmitted by a slave base station andreceiving in a first retransmission the second set of data retransmittedby the master base station.

In some embodiments, the method further comprises receiving in the firstretransmission the first set of data retransmitted by the slave basestation.

In some embodiments, receiving the retransmitted second set of data fromthe master base station occurs synchronously with receiving theretransmitted first set of data from the slave base station.

In some embodiments, receiving the retransmitted second set of data fromthe master base station occurs asynchronously with receiving theretransmitted first set of data from the slave base station.

In accordance with still another broad aspect, there is provided amultiple-input, multiple-output (MIMO) communication system comprising amaster base station having a plurality of transmit antennas andconfigured to transmit a first set of data to a mobile station in afirst transmission and a slave base station having a plurality oftransmit antennas and configured to transmit a second set of data to themobile station in the first transmission. The master base station isfurther configured to retransmit the second set of data to the mobilestation during a first retransmission.

In some embodiments, the slave base station is further configured toretransmit the first set of data to the mobile station during the firstretransmission.

In some embodiments, the master base station retransmits the second setof data synchronously with the slave base station retransmitting thefirst set of data.

In some embodiments, the master base station retransmits the second setof data asynchronously with the slave base station retransmitting thefirst set of data.

In accordance with yet another broad aspect, there is provided a methodin a multiple-input, multiple-output (MIMO) communication system. Themethod comprises in a master base station, transmitting a first set ofdata to a mobile station in a first transmission; in a slave basestation, transmitting a second set of data to the mobile station in thefirst transmission; and in the master base station, retransmitting thesecond set of data to the mobile station during a first retransmission.

In some embodiments, the method further comprises, in the slave basestation, retransmitting the first set of data to the mobile stationduring the first retransmission.

In some embodiments, retransmitting the second set of data to the mobilestation during the first retransmission occurs synchronously withretransmitting the first set of data to the mobile station during thefirst retransmission.

In some embodiments, retransmitting the second set of data to the mobilestation during the first retransmission occurs asynchronously withretransmitting the first set of data to the mobile station during thefirst retransmission.

Other aspects and features of the present invention will becomeapparent, to those ordinarily skilled in the art, upon review of thefollowing description of the specific embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe accompanying diagrams, in which:

FIG. 1 depicts a MIMO communication system for network MIMOtransmissions in accordance with an embodiment;

FIG. 2 depicts transmission of a look-up table of possible combinationsof network MIMO groups for a mobile station;

FIG. 3 depicts a representative example of one possible frame diagram ofnetwork MIMO with backhaul communication;

FIG. 4 depicts a representative example of one possible frame diagram ofmobile-assisted network MIMO communication;

FIG. 5 is an example schematic diagram of three network MIMO zones foreach BS;

FIGS. 6 and 7 depict exemplary scattering of pilot symbols;

FIG. 8 depicts a diagram of a HARQ (Hybrid Automatic Repeat Request)re-transmission scheme;

FIG. 9 is a plot relating to network MIMO precoding;

FIG. 10 is a flowchart of a method, in a system, of enabling networkMIMO among a plurality of base stations and at a least one mobilestation; and

FIG. 11 is a flowchart of a method, in a mobile station, of enablingnetwork MIMO among a plurality of base stations and at a least onemobile station.

DETAILED DESCRIPTION OF THE INVENTION

In a MIMO system, a base station (BS) provides communication servicesfor a coverage area or cell in a wireless communication system. The term“base station” can refer to any access point providing coverage to anarea. The BS transmits communication signals to mobile stations (MSs)via multiple antennas. MSs are also commonly referred to as userterminals, user equipment, and communication devices, for instance. Theterm “mobile station” can refer to any receiving device (stationary ormobile). At a MS side, multiple receive antennas are employed for eachMS.

FIG. 1 shows a MIMO communication system 10 for network MIMO. MIMOcommunication system 10 includes BSs BS_(A) 100, BS_(B) 102, BS_(C) 104and MSs MS₁ 106, and MS₂ 108. Each of MS₁ 106 and MS₂ 108 can be awireless device, such as a cellular telephone, a computer with awireless modem, or a PDA (personal Digital Assistant).

Each of BS_(A) 100, BS_(B) 102, BS_(C) 104 serves a particular cell tofacilitate communication with one or more MSs located within the cellassociated with the corresponding BS. MS₁ 106 is linked to BS_(A) 100,and MS₂ 108 is linked to BS_(C) 104. Thus, BS_(A) 100 and BS_(C) 104 arethe serving BSs for MS₁ 106 and MS₂ 108, respectively. MS₁ 106 islocated at the intersection of two cells, as is MS₂ 108. As such, theyare known as “cell-edge” MSs. It is to be understood that cell-edge MSsare a particular example, and embodiments are not limited to cell-edgeMSs.

In operation, network MIMO combines antennas from BSs located inneighboring cells to transmit multiple streams to one or more MSs. Thus,each MS is in network MIMO communication with multiple BSs, includingits serving BS. More particularly, in network MIMO MS₁ 106 can be innetwork MIMO communication with BS_(A) 100 and either BS_(B) 102 orBS_(C) 104. MS₂ 108 can be in network MIMO communication with BS_(C) 104and either BS_(A) 100 or BS_(B) 102.

In order to support network MIMO, various features are provided by theembodiments described herein: network MIMO co-ordination, feedback,network MIMO with backhaul communications, mobile-assisted network MIMO,network MIMO zone, HARQ re-transmission, and precoded network MIMO.These features are described below.

The following describes network MIMO co-ordination. As noted above, innetwork MIMO each MS is in network MIMO communication with multiple BSsTo determine which BSs are in network MIMO communication with aparticular MS, each MS is provided with an “active set.” The active setof a MS indicates eligible BSs for network MIMO transmission with thatparticular MS. The active set may be stored or updated by a MS from timeto time. In the example of FIG. 1, MS₁ 106 and MS₂ 108 each have anactive set that indicates that BS_(A) 100, BS_(B) 102, and BS_(C) 104are eligible for network MIMO transmission.

An active set can be based on any number of considerations, such as thebest BSs to be used from a signal strength, proximity, or interferencestandpoint. For example, the active set of a MS can be based on thesignal strength of the preamble of a signal received by the MS. It is tobe understood that generation of the active set is not limited to theseconsiderations.

Generally, multiple BSs participating in network MIMO with a MS can becollectively referred to as a “network MIMO group” for a particular MS.The network MIMO group can be a subset of the MS's active set. BSsparticipating in network MIMO transmission can be referred to as“participating BSs.” In FIG. 1, MS₁ 106 has a network MIMO group 112consisting of BS_(A) 100 and BS_(B) 102. MS₂ 108 has a network MIMOgroup 114 consisting of BS_(B) 102 and BS_(C) 104. Although in FIG. 1the network MIMO group of MS₁ 106 and MS₂ 108 is a subset of the MS'sactive set, this does not have to be the case.

The active set can indicate BSs eligible for network MIMO download andupload. If such download and upload active sets are used, the networkMIMO group contains all or a subset of the BSs that are the union of thedownload and upload active sets. The upload and download groups can bedifferent or the same.

As shown in FIG. 1, BS_(A) 100 has a control system 115 which is adaptedto configure a network MIMO zone. Network MIMO zones are described inmore detail below with reference to FIG. 5. Each BS can have a controlsystem.

An MS determines a network MIMO group based on its active set. An MS maydetermine the network MIMO group using a look-up table of possiblecombinations of network MIMO groups, based on the MS's active set. Manycombinations are possible. FIG. 2 depicts transmission of a look-uptable 116 of possible combinations of network MIMO groups for MS₁ 106.The second column of the look-up table 116 defines a combination ofnetwork MIMO groups, and the first column defines indices such that eachindex is associated with a respective combination. Various indexingschemes are possible. According to one indexing scheme, if there is amaximum of N BSs in the network MIMO group, then the look-up tablecontain combinations of the N−1 BSs, excluding the serving BS. Thiskeeps the number of possible combinations smaller, such that thecombinations can be signaled with less overhead. However, thecombinations in the table could be extended to include the serving BS.If the serving BS is excluded from the combinations, the number of rowsin the look-up table would be 2^((N−1))−1. Such an indexing scheme hasbeen used for the look-up table 116. Referring to FIG. 2, MS₁ 106 canhave a maximum of 3 BSs in any of its network MIMO groups, so thelook-up table 116 contains combinations of 3−1=2 BSs, excluding servingBS_(A) 100. Look-up table 116 has 2⁽³⁻¹⁾−1=3 rows. Referring to thefirst row, a network MIMO group consisting of BS_(B) 102 is associatedwith index “0”. Referring to the second row, a network MIMO groupconsisting of BS_(C) 104 is associated with index “1”. Referring to thethird row, a network MIMO group consisting of BS_(B) 102 and BS_(C) 104is associated with index “2”.

MS₁ 106 generates the look-up table 116 based on its active set. Thelook-up table 116 is stored at MS₁ 106 and its serving BS_(A) 100.Alternatively, the look-up table 116 is generated independently atserving BS_(A) 100 by following certain rules. For example, if MS₁ 106sends a network MIMO group in a certain order, the look-up table can begenerated according to such order. Although in this example the rule forgenerating the table depends on the order in which MS₁ 106 reports thenetwork MIMO group, other rules may be used.

After generating the look-up table 116, MS₁ 106 determines a networkMIMO group and can transmit an index associated with the network MIMOgroup. The index can be transmitted on an uplink (UL) feedback channel.Transmitting the index can require N−1 bits, which may not be large inpractical network MIMO communication systems.

In the example of FIG. 2, if MS₁ 106 decides to receive network MIMOtransmissions from BS_(A) 100 and BS_(B) 102, then MS₁ 106 looks up thecombination BS_(B) 102 in the look-up table 116, notes that thiscombination is associated with index “1”, and transmit index “1”. MS₁106 transmits the index to its serving BS_(A) 100, which relays theindex to the remaining BSs in the network MIMO group, namely BS_(B) 102and BS_(C) 104 (illustrated using solid lines). Alternatively, MS₁ 106broadcasts the index to the network MIMO group, namely BS_(A) 100 andBS_(B) 102 (illustrated using broken lines). The index may betransmitted on a UL feedback channel. In addition to an index, MSidentification data, such as a MS identifier (ID), can also betransmitted to a BS in order to allow that BS to decode information fromand relay information to the MS. Whenever the network MIMO group of a MSchanges, for example due to a change in the active set of that MS, itsserving BS sends the updated look-up table to the remaining BSs in theupdated active set.

Upon receipt of the index from MS₁ 106, a BS retrieves the correspondingnetwork MIMO group, and prepares for network MIMO accordingly. Forexample, BS_(B) 102 will note that it is listed in the network MIMOgroup and prepares for network MIMO with MS₁ 106, while BS_(C) 104 notesthat it is not listed in the network MIMO group and therefore does notprepare for network MIMO with MS₁ 106.

The above described network MIMO co-ordination, including determining anetwork MIMO group based on an active set. Before such determination, aMS can be involved in feedback to assist in making that determination.The following describes such feedback.

MSs that are eligible for network MIMO are sent an indication ofeligibility for network MIMO. Such indication can be sent to MSs withina cell, cell-edge MSs, or both.

Upon receiving an indication of network MIMO, a MS measures C/I (Carrierto Interference Ratio) based on pilots the MS has received.

The MS determines whether it is ready to receive network MIMOtransmission. The determination can be based on various factors. Threeexample factors are described below.

The determination can be based on an absolute C/I threshold and/or adifference in the C/Is of a MS's neighboring BSs, since the MS has fullknowledge of the channel conditions of the BSs in the active set.

The determination can be based on instantaneous or average channelconditions. In this case, a subset of the BSs could be in the activeset.

The determination can be based on the MS's receiver structure, forexample Minimum Mean-Squared Error (MMSE) or Successive InterferenceCancellation (SIC) with MMSE. With SIC, the MS can decide to do networkMIMO when three is a larger differential in the C/I between serving BSand the remaining BSs in its active set than in the case where SIC isnot used.

If the MS determines that it is not ready, the MS does not send anetwork MIMO indication. IF the MS determines that it is ready, the MSsends feedback information, including a network IMO indication,information on the C/Is, an index, and an indication of MIMO mode. MIMOmode refers to the type of MIMO transmission, such as open-loop,closed-loop, blast, SM (spatial multiplexing), and STTD (space-timetransmit diversity). The MS can also send a preferred matrix index toindicate its choice of precoding.

The MS sends the feedback to all BSs in the network MIMO group, or asubset of it.

The serving BS designates UL resources for the feedback information. Theserving BS signals the location of the UL resources to the network MIMOgroup so that the network MIMO group can retrieve the feedbackinformation. Since only a subset of the BSs in the network MIMO groupmay be involved in an actual network MIMO transmission, the BS sends theindex on the uplink.

Since the uplink and downlink active sets may be different, the ULfeedback can be sent in two ways. If the participating BSs are in theuplink active set, the UL feedback channel can be adjusted so that ittargets the participating BSs to hear the feedback. For example, if theparticipating BSs are in the uplink active set (e.g. in TDD (TimeDivision Duplex)), the UL feedback channel can be adjusted in terms ofpower. If the participating BSs are not in the uplink active set, theserving BS is responsible for decoding the UL feedback.

The BS can encode transmissions vertically or horizontally, as describedbelow.

For vertical encoding or STTD, on C/I is reported. The C/I channel isscrambled by the MS ID. Since one encoded packet can be sent to multiplestreams in vertical encoding, the participating BSs can encode the samedata independently (since they have already been receiving copies of theMS data) and extract the portion of the encoded data for transmission.Alternatively, the serving BS encodes the data and sends the portion ofthe encoded data to the BSs in the network MIMO.

For horizontal encoding, C/Is for different streams are reported foreach participating BS. A C/I channel can be scrambled by the MS ID. TheBSs that will participate in the network MIMO transmission to the MS maybe signaled by the index of the look-up table. The order of the C/Ireport for each stream (whether it is encoded separately or jointly)corresponds to the order in the entry of the look-up table.

The serving BS performs Modulation and Coding Scheme (MCS) selectionsbased on the reported C/Is and signals the other participating BSs andMCS. Alternatively, MCS selection can be performed independently by theparticipating BSs.

FIG. 3 depicts a representative example of one possible frame diagram ofnetwork MIMO with backhaul communication. MS₁ 106, BS_(A) 100 and BS_(B)102 can communicate with each other over frames N to N+8. As notedabove, MS₁ 106 measures C/Is for the network MIMO. MS₁ 106 feedbacks theC/Is to the serving BS_(A) 100. Serving BS_(A) 100 can send to the otherparticipating Bs, namely BS_(B) 102, scheduling information, whichincludes information on resource allocation, the MCS, the MIMO mode, andthe transmission time (e.g. frame number with the appropriate offset forthe different BSs). Alternatively, the scheduling can be performed byeach participating BS individually.

Serving BS_(A) 100 can also send an indication of eligibility fornetwork MIMO.

BS_(B) 102 can send an indication of participation in the network MIMO,for instance an acknowledgement (e.g. an ACK) to indicate participation,or a negative acknowledgement (e.g. a NACK) to indicate no participationfor example due to loading in the BSs.

If serving BS_(A) 100 does not receive a response from BS_(B) 102, thenBS_(A) 100 can assume that the indication of eligibility was notreceived by BS_(B) 102. In that case, BS_(A) 100 can resend theindication of eligibility.

Where there are only two BSs in the network MIMO group, such as BS_(A)100 and BS_(B) 102, serving BS_(A) 100 can change the MIMO mode ifBS_(B) 102 does not respond or a NACK is received. For example, an STTDrate 2 can be changed to STTD rate 1.

Finally, BS_(A) 100 or BS_(B) 102 transmits control signaling data andnetwork MIMO data to MS₁ 106.

FIG. 4 depicts a representative example of one possible frame diagram ofmobile-assisted network MIMO communication, as an alternative to thenetwork MIMO with backhaul communications of FIG. 3. Mobile-assistednetwork MIMO uses signaling between BSs and MSs to enable network MIMOtransmissions.

Mobile-assisted network MIMO eliminates backhaul signaling among BSs. Itdeals with network MIMO when at least one participating BS is fromanother cell.

The MS is used relay information. Together with short frame duration,mobile-assisted Network MIMO reduces the scheduling delay and enablesmore dynamic scheduling.

In every superframe, a set of resources for network MIMO transmission,known as a network MIMO zone, is configured. Network MIMO zones aredescribed in more detail below with reference to FIG. 5. ParticipatingBSs are determined based on MS feedback.

Referring to FIG. 4, the steps of MS₁ 106 measuring C/Is for the networkMIMO and feeding back the C/Is to the master BS_(A) 100 are the same asin FIG. 3.

Afterwards, the serving BS_(A) 100 schedules MS₁ 106 in the network MIMOzone using a scheduler. A fixed amount of time is allowed to elapsebetween this decision of the scheduler and actual transmission, forinstance 5 frames from N+4 to N+8 in the example of FIG. 4.

BS_(A) 100 sends scheduling information to MS₁ 106, including resourceallocation, MCS, and MIMO mode. At frame N+5, MS₁ 106 relays thescheduling info to the participating BSs, such as BS_(B) 102.

BS_(B) 102 sends an indication of participation in the network MIMO,such as an ACK, to MS₁ 106, so that MS₁ 106 can be ready to decode datafrom BS_(B) 102. This allows MS₁ 106 to determine how many participatingBSs successfully receive the indication of eligibility for network MIMO.If MS₁ 106 does not receive an ACK/NACK from BS_(B) 102, it assumes thatthe indication of eligibility for network MIMO was not received byBS_(B) 102. The MS₁ 106 sends the scheduling info through higher-layersignaling to re-synchronize the data.

Finally BS_(A) 100 and BS_(B) 102 transmit data to MS₁ 106.

FIG. 5 is an example schematic diagram of three network MIMO zones foreach BS. Generally, resources for network MIMO transmission are referredto as a “network MIMO zone”. The network MIMO zone can be described as aset of two dimensional resources (time and frequency), though in someembodiments code spreading can be used to provide a third dimension. Thenetwork MIMO zone can be a TDM (Time Divisional Multiplexing)-basedzone, an FDM (Frequency Division Multiplexing)-based zone, or a combinedTDM/FDM-based zone.

Referring to FIG. 5, shown are resources 117,118,119 for BS_(B) 102, andBS_(C) 104, respectively. These resources are shown as having a twodimensional appearance in which the horizontal direction is frequencyand the vertical direction is time. The resources for each BS arepartitioned into 3 zones, which can be used for network MIMOtransmission. In the example of FIG. 5, a MS is in network MIMOcommunication with BS_(A) 100, BS_(B) 102, and BS_(C) 104 of FIG. 1.

Shown are various types of partitions. For each resource, a BS known asthe “master” BS transmits, and other participating BSs transmit as“slaves”. For illustration purposes, a network MIMO zone assigned to amaster for network MIMO transmission is illustrated with dense stripesor dotting. A network MIMO zone assigned to a slave for network MIMOtransmission is illustrated with light stripes or dotting. A networkMIMO zone assigned to a BS for non-network-MIMO transmission isillustrated with blank space. If there is no data transmission in anetwork MIMO zone, non-network MIMO transmission can be scheduled toavoid wastage of resources.

More specifically, resource 117 has a network MIMO zone 120 assigned toBS_(A) 100 as master for network MIMO transmission, a network MIMO zone122 assigned to BS_(B) 102 for non network MIMO transmission, and anetwork MIMO zone 124 assigned to BS_(C) 104 as slave for network MIMOtransmission. On network MIMO zone 120, BS_(A) 100 can transmit data toa network MIMO MS (not shown). BS_(A) 100 can transmit, on network zone122, data to a non-network MIMO MS, i.e. a MS not in network MIMO withBSs BS_(A) 100, BS_(B) 102, and BS_(C) 104. BS_(A) 100 can transmit, onnetwork MIMO zone 124, data that BS_(C) 104 has instructed it totransmit, to the network MIMO MS.

Resource 118 has a network MIMO zone 126 assigned to BS_(A) 100 as slavefor network MIMO transmission, a network MIMO zone 128 assigned toBS_(B) 102 as master for network MIMO transmission, and a network MIMOzone 130 assigned to BS_(C) 104 for non network MIMO transmission.Resource 119 has a network MIMO zone 132 assigned to BS_(A) 100 for nonnetwork MIMO transmission, a network MIMO zone 134 assigned to BS_(B)102 as slave for network MIMO transmission, and a network MIMO zone 136assigned to BS_(C) 104 as master for network MIMO transmission.

The same channelization procedure and hopping pattern arrangement can beused. Thus, the allocation of sub-resources to users in the same for allof the network MIMO zones for a network MIMO group. However, when nonetwork MIMO is scheduled, BS specific channelization procedure andhopping pattern can be used.

The size of the network MIMO zone can be configured every superframebased on the number of network MIMO users.

FIGS. 6 and 7 depict exemplary scattering of pilot signals. BSs sendpilot signals, which the MSs receive an use for channel estimation.Common pilots that are orthogonal can be used. This can facilitatechannel estimation and precoder selection for closed loop network MIMO.

The pilots can be sent for all antennas, or for a subset of antennaswith cycling. Such pilot subset cycling can reduce pilot overhead.Alternatively, other pilots could be used, for example dedicated pilots.

FIG. 6 depicts an embodiment where 2 BSs send pilots for all antennas.Shown on the left side is a network MIMO zone in which pilots for all ofthe two antennas of BS_(A) 100 of FIG. 1 have been scheduled, namelypilots 140,142,144,146. Shown on the right side is a network MIMO zonein which pilots for all of the two antennas of BS_(B) 102 of FIG. 1 havebeen scheduled, namely pilots 148,150,152,154.

In operation, participating BSs send pilots to a MS. The MS receives thepilots, measures the C/Is for the pilots, estimate channels for allantennas, and report on the C/Is.

The MS can be configured to receive data transmission from the antennasof the BSs and report on their C/Is in any number of ways. Morespecifically, the MS can be configured to receive data transmission fromall antennas, data transmission with antenna hopping, or datatransmission with antenna selection.

In the case of data transmission from all antennas, the MS reports oneor multiple C/Is for all antennas, and BSs transmit data from allantennas. Transmission at an STTD (Space-time block coding basedtransmit diversity) rate 2 can be used.

In the case of data transmission with antenna hopping, the MS reportsone or multiple C/Is for all antennas, and BSs transmit data on a subsetof the antennas with a pre-defined hopping pattern which hops around allantennas. Transmission at an STTD rate 1 with antenna hopping can beused.

In the case of data transmission with antenna selection, the MS reportsone or multiple C/Is for a subset of antennas. The BSs transmit data onthe subset of antennas.

Since pilots are sent for all transmit antennas and the MS needs toestimate channels for all antennas, the embodiment of FIG. 6 canrepresent a higher overhead and computation complexity. However, it canprovide full flexibility to achieve spatial diversity.

FIG. 7 depicts an embodiment where to BSs send a subset of pilots andcycle the pilots in a regular interval. Only a subset of pilots is senton a network MIMO zone to reduce the pilot overhead. More specifically,BS_(A) 100 of FIG. 1 sends a subset consisting of pilots 140,142, andlater a subset consisting of pilots 144,146. BS_(B) 102 of FIG. 1 sendsa subset consisting of pilots 140,142, and later a subset consisting ofpilots 144,146.

Pilots are cycled in a pre-defined pattern in a regular interval foradditional spatial diversity. In order to enable proper C/I reportingand channel estimation for data demodulation, the pilot cycling patterncan be configured to be changed only every superframe so that within thesuperframe, a MS reports and estimates channels from the same set ofpilots.

The MS measures the C/Is from all pilots.

As with the embodiment of FIG. 6, in FIG. 7 the MS can be configured forreceiving data transmission from all antennas, data transmission withantenna hopping, or data transmission with antenna selection.

FIG. 8 depicts a diagram of a HARQ re-transmission scheme.Re-transmission can be either synchronous or asynchronous.

A network MIMO resource assignment can be persistent until either thepacket is correctly received or N packets are correctly received. A MSmay have enough data for consecutive transmissions without additionalsignaling.

The data can be cycled through the BSs in re-transmission for additionaldiversity such that the MS receives all the data in subsequentre-transmissions even if only the serving BS is transmitting, forexample in STTD rate 2,4. Instead of STTD, Space-Time Coding (STC) couldbe used.

In the example of FIG. 8, in an initial transmission, a first set ofdata (shown as symbols s₁, s₂, −s₁*, and s₁*) is sent by a master BS anda second set of data (shown as symbols s₃, s₄, −s₄*, and s₃*) is sent bya slave BS. In the first re-transmission, the second set of data is sentby the master BS and the first set of data is sent by the slave BS.

As noted above in respect of FIG. 2, the BS can encode transmissionsvertically or horizontally.

In vertical encoding, one ACK/NACK is used for each transmission. The MSdecodes the data successfully and sends an ACK to the BSs. All BSsreceive the ACK. BSs can schedule other users in the resource. Where atleast one BS does not receive the ACK or mistake it as a NACK, these BSsretransmit the data and do not hear the ACK/NACK again. In other words,they abort re-transmission.

Where the MS is unable to decode the data, the MS sends a NACK as anACK, the BS schedules non-network MIMO users in a network MIMO zone. TheMS may still soft-combine all data, but half of the data will becorrupted. To resolve this issue, a re-transmission indicator (e.g. 1bit) can be sent from each BS to indicate the presence ofre-transmission from that particular BS. For example, the MS onlysoft-combine data from BSs with the indicator set to 1. Alternatively,the MS blindly detects the received signal (after soft-combining) byassuming that the received signal either contains the re-transmission ornot.

In horizontal encoding, on ACK/NACK is used for each layer. The sectionsof data are cycled for redundancy. When one layer finishesre-transmission, it can retransmit the data in another layer so that itcan be soft combined for diversity (e.g. SFN transmission). For example,in a 2-layer spatial multiplexing, when the first layer is receivedsuccessfully while the other layer is not, the same data for the 2^(nd)layer can be transmitted on the first layer in subsequentre-transmissions.

The following describes precoding in network MIMO. In a network MIMOzone, orthogonal common pilots facilitate channel estimation of thenetwork MIMO channel and joint precoder selection. The joint precodercan be selected in many ways, for example by the serving BS, the otherparticipating BSs, or the MS. The serving BS can select the jointprecoder based on sounding. The MS can select the joint precoder basedon feedback.

In codebook-based precoding, the network MIMO MS determines a preferredprecoder.

Participating BSs obtain precoder feedback via a UL feedback channel,and may need to confirm the choice of precoder via backhaul handshake.

If the precoder is a matrix of size N_(tx) _(—) _(total)×N_(streams),and each BS has N_(tx) transmit antennas, then the precoder is dividedinto blocks of an N_(tx)×N_(streams) submatrix with the master BS usingthe first block, and the second BS using the second block. The order isalready determined by the look-up table 180, and the index is signaledby the MS as described in respect of FIG. 2.

To further illustrate network MIMO, a very specific example of a networkMIMO communication system is set forth below.

In a network MIMO zone, the pilot pattern used can be a four antennapattern in accordance with C802.16m-08/172r1. Each BS transmits pilotsfor 2 different antennas.

The HARZ re-transmission is asynchronous, synchronous, or RAS-HARQ.

A permutation index can be used to signal the resource partition withinthe network MIMO zone in accordance with C802.16m-08/172r1.

A diversity zone or a localized zone is used, as described below.

In a diversity zone, a network MIMO zone is defined by using the samechannelization procedure as for Fractional Frequency Reuse (FFR). A FFRzone corresponding to reuse one is used for network MIMO. A commonhopping pattern is used by the coordinating BSs in this zone. If thereis no MS eligible for network MIMO transmission, BS specific hoppingpattern is used and non-network MIMO MSs are scheduled.

In a localized zone, localized zones between coordinating BSs arephysically aligned. Network MIMO is transparent to the user in the caseof asynchronous HARQ. In synchronous HARQ or RAS-HARQ, only the timingof the re-transmissions is different in network MIMO to account for thedelay associated with coordinating the transmissions. The C/Imeasurement pilots are located on the same tones as in the case of anetwork MIMO zone for non-network MIMO transmission. The controlinformation is the same as in the network MIMO zone for non-network MIMOtransmission.

In terms of procedure, a BS configures a network MIMO zone with aneighboring BS. The location of the network MIMO zone, the coordinatingBS ID, and the hopping pattern are signaled in a superframe header. TheBS schedules a user in the network MIMO zone. The BS coordinates variousaspects with the participating BSs supporting the serving BS, includinguser election and resource assignment. The BS sends control informationand transmission data to the MS. Re-transmission can occur either insideor outside the network MIMO zone.

A MS reports its active set to its serving BS, which can be based onsignal strength, or a static determination. This indicates which BSs canbe used for network MIMO transmission. For an open loop, the MS measuresand reports the C/I for STTD or SM (spatial multiplexing) to the servingBS. For a closed loop, the MS measures and reports a precoding matrixindex (PMI), a rank and C/I to the serving BS. The rank generally refersto the number of streams that the MS is able to receive. The MS decodesthe control and transmitted data, and sends an ACK/NACK to the servingBS.

FIG. 9 is a plot relating to network MIMO precoding. The plot showscumulative distribution functions (CDF) against Signal to Noise Ratios(SNR) for various scenarios). FIG. 9 shows the possible gain or networkMIMO over 2×2 CL (closed loop) MIMO and 4×2 CL MIMO. It also shows thegain over the case where the most dominant interferer is nottransmitting.

FIG. 10 is a flowchart of a method 180, in a system, of enabling networkMIMO among a plurality of BSs and at a least one MS. Step 182 involvesconfiguring at a BS a network MIMO zone based on an indication of atleast one other BS eligible for network MIMO. The network MIMO zone isdefined by resources having at least time and frequency dimensionsallocated for master transmission under control of the BS, or slavetransmission under control of one of the at least one other BS eligiblefor network MIMO.

FIG. 11 is a flowchart of a method 200, in a MS, of enabling networkMIMO among a plurality of BSs and at a least one MS. Step 202 involvesdetermining at the MS BSs eligible for network MIMO. Step 204 involvestransmitting an indication of the BSs eligible for network MIMO to atleast one of the BSs eligible for network MIMO. Step 206 involvesreceiving data on a network MIMO zone from at least one antenna of a BSof the BSs eligible for network MIMO. The network MIMO zone is definedby a resource allocated for master transmission under control of the BS,or slave transmission under control of another BS eligible for networkMIMO.

What has been described is merely illustrative of the application of theprinciples of the invention. Other arrangements and methods can beimplemented by those skilled in the art without departing from thespirit and scope of the present invention.

What is claimed is:
 1. A method of enabling network Multiple-InputMultiple-Output (MIMO) among a plurality of Base Stations (BSs) and aMobile Station (MS), the method comprising: determining at a first ofthe plurality of BSs that the MS is eligible for MIMO; sending to the MSan indication of network MIMO, wherein the indication of network MIMOcauses the MS to measure Carrier to Interference Ratios (C/Is) based onreceived pilot signals; receiving feedback information from the MS,wherein the feedback information comprises at least one of the BSseligible for MIMO, the C/Is based on the pilot signals, a precodingmatrix index, and a rank indicating streams for receiving data.
 2. Themethod of claim 1, further comprising sending the pilot signals to theMS.
 3. The method of claim 1, wherein the indication of network MIMOfurther causes the MS to determine whether the MS is ready to receive anetwork MIMO transmission.
 4. The method of claim 3, wherein thedetermination of whether the MS is ready to receive the network MIMOtransmission is based upon at least one of an absolute C/I threshold, adifference in the C/Is between the first of the plurality of BSs and asecond of the plurality of BSs that is eligible for MIMO, instantaneouschannel conditions, and the structure of the MS.
 5. The method of claim1, wherein the feedback information comprises an identification of anetwork MIMO group comprising the first of the plurality of BSs and asecond of the plurality of BSs.
 6. The method of claim 5, furthercomprising configuring a network MIMO zone based on the identificationof the network MIMO group.
 7. A base station (BS) in a Multiple-InputMultiple-Output (MIMO) system for enabling network MIMO among aplurality of BSs and at least one mobile station (MS), the BScomprising: at least one transmitting antenna; and a control systemconfigured to: determine that the MS is eligible for MIMO; send to theMS an indication of network MIMO, wherein the indication of network MIMOcauses the MS to measure Carrier to Interference Ratios (C/Is) based onreceived pilot signals; receive feedback information from the MS,wherein the feedback information comprises at least one of the BSseligible for MIMO, the C/Is based on the pilot signals, a precodingmatrix index, and a rank indicating streams for receiving data.
 8. TheBS of claim 7, wherein the control system is further configured to sendthe pilot signals to the MS.
 9. The BS of claim 7, wherein theindication of network MIMO further causes the MS to determine whetherthe MS is ready to receive a network MIMO transmission.
 10. The BS ofclaim 9, wherein the determination of whether the MS is ready to receivethe network MIMO transmission is based upon at least one of an absoluteC/I threshold, a difference in the C/Is between the first of theplurality of BSs and a second of the plurality of BSs that is eligiblefor MIMO, instantaneous channel conditions, and the structure of the MS.11. The BS of claim 7, wherein the feedback information comprises anidentification of a network MIMO group comprising the first of theplurality of BSs and a second of the plurality of BSs.
 12. The BS ofclaim 11, wherein the control system is further configured to configurea network MIMO zone based on the identification of the network MIMOgroup.